Diffractive optical element, optical system having the same, and imaging apparatus

Abstract

A diffractive optical element includes a first lens having a convex surface, a second lens having a concave surface, disposed in such a manner that the concave surface of the second lens faces the convex surface of the first lens, and a diffraction grating section formed between the first and the second lenses and having positive optical power through diffraction. The diffraction grating section includes a first diffraction grating and a second diffraction grating disposed in this order from a side closer to the first lens; the second diffraction grating has a refractive index larger than that of the first diffraction grating, and an inner diameter of a grating wall surface of the diffraction grating section decreases as approaching to the second lens from the first lens.

What is claimed is: 
     
       1. A An optical system comprising a diffractive optical element, wherein the diffractive optical element comprises, in an order from an object side to an image side:
 a first lens having a convex surface; 
 a diffraction grating section having positive optical power through diffraction; and 
 a second lens having a concave surface facing the convex surface, 
 wherein the diffraction grating section includes a first diffraction grating and a second diffraction grating disposed in this order from the object side to the image side, the second diffraction grating having a larger refractive index than the first diffraction grating, 
 wherein the diffraction grating section has a plurality of grating surfaces that contributes to the diffraction and a plurality of grating wall surfaces that is adjacent to the plurality of grating surfaces, and 
 wherein an inner diameter of a first grating wall surface of the first diffraction grating among the plurality of grating wall surfaces decreases from the object side to the image side. 
 
     
     
       2. The optical system according to  claim 1 , wherein the first lens is a positive lens. 
     
     
       3. The optical system according to  claim 1 , wherein the second lens is a negative lens. 
     
     
       4. The optical system according to  claim 1 , wherein the following conditional expression is satisfied:
   20<ν LL −ν LR <60
 
 where ν LL  is an Abbe number of the first lens, and ν LR  is an Abbe number of the second lens. 
 
     
     
       5. The optical system according to  claim 1 , wherein the following conditional expression is satisfied:
   0.8< N   L   /N   LL <1.2 
 where N LL  is a refractive index of the first lens, and N L  is a refractive index of the first diffraction grating. 
 
     
     
       6. The optical system according to  claim 1 , wherein the following conditional expression is satisfied:
   5 degrees<Δθ HM <45 degrees
 
 where Δθ HM  is a difference between maximum and minimum values of an angle θ HM  in the diffraction grating section, θ HM  being defined as an absolute value of an angle formed by each of the grating wall surfaces and a surface normal to an enveloping surface of the diffraction grating section, formed by connecting apical portions of the first diffraction grating, at positions where each of the plurality of grating wall surfaces contacts the enveloping surface. 
 
     
     
       7. The optical system according to  claim 1 , wherein at least one of the first and the second diffraction gratings is formed of resin. 
     
     
       8. The optical system according to  claim 1 , further comprising: an aperture diaphragm disposed on an image side of the optical element. 
     
     
       9. The optical system according to  claim 1 , further comprising:
 a partial optical system including lenses disposed on the object side of the diffractive optical element, 
 wherein the partial optical system has positive refractive power. 
 
     
     
       10. The optical system according to  claim 1 , wherein the following conditional expression is satisfied:
   0.10< L   d   /L   t <0.50 
 where L d  is a distance on the optical axis from a lens surface of the first lens on a side where the diffraction grating section is formed to a lens surface of the optical system on the most object side, and L t  is a total length of the optical system. 
 
     
     
       11. The optical system according to  claim 1 , wherein the following conditional expression is satisfied:
   −2.0< f /( R   d   ×Fn )<−0.20
 
 where f is a focal length of the optical system, Fn is an open F-number of the optical system, and R d  is a curvature radius of a lens surface of the first lens on a side where the diffraction grating section is formed. 
 
     
     
       12. The optical system according to  claim 1 , wherein the following conditional expression is satisfied:
   0.6<( E   1   −E   d )/ L   d   +E   d   ×P   f   −E   d   /R   d <2.0 
 where E 1  is an effective diameter of a lens surface of the optical system on the most object side, E d  is an effective diameter of a lens surface of the first lens on a side where the diffraction grating section is formed, L d  is a distance on the optical axis from the lens surface of the first lens on the side where the diffraction grating section is formed to the lens surface of the optical system on the most object side, R d  is a curvature radius of the lens surface of the first lens on the side where the diffraction grating section is formed, and P f  is refracting power of a lens surface of the first lens on a side where the diffraction grating section is not formed. 
 
     
     
       13. The optical system according to  claim 1 , wherein the following conditional expression is satisfied:
   10 degrees<|θ D |<57 degrees
 
 where θ D  is an incident angle of an axial marginal ray incident on the diffraction grating section in a state of being focused at infinity. 
 
     
     
       14. An imaging apparatus comprising:
 the optical system according to  claim 1 ; and 
 an image sensor configured to receive light of an image formed by the optical system.